In the digital printing of color images, image-based digital data causes placement of different colorants at specific locations on a print sheet. These colorants, such as toner or liquid ink, typically relate to primary colors which are blended together on the print sheet to yield a realistic image. Because a typical set of colorants would be cyan, magenta, and yellow (CMY), it follows that the image data, on a pixel-by-pixel basis, would be in a CMY color space.
Signals for television or video tape transmission, however, generally do not operate in a CMY or similar color space. Rather, there are provided separate luminance and chrominance signals. The luminance signal controls the overall brightness of a particular portion of an image being rendered, and one or more chrominance signals are used to indicate coloration of a particular area of the image. In the color signal specified by the National Television System Committee (NTSC), the chrominance signal is superimposed on a high-frequency region of the luminance signal; in general the purpose of this arrangement is to allow color original signals to be received on a black-and-white receiver.
Although there exists in the prior art any number of proposed systems for improving color image quality from a luminance-chrominance video signal, luminance-chrominance signals have many practical drawbacks relating to the fact that the chrominance signal is restricted to a high-frequency region of the luminance signal. One such drawback is that chrominance signals, as one scans signals relating to pixels in one frame of a video image, cannot change as fast as luminance signals. One result of this is the artifact of "color bleed." If one looks at a video image in an area where there is a sharp change from a bright (high luminance) area to a relatively dark area, very often one will see a colored blur around the border between the dark and light area. This color bleed occurs because the chrominance signal cannot change as abruptly as the luminance signal through the image.
In most applications of moving video pictures, the low inherent resolution of a television image coupled with the fact the images are moving tends to make this color bleed fairly inconspicuous. However, in an apparatus which can capture individual frames from a television or video tape signal, and print out the captured image digitally, such as with an ink-jet or electrophotographic printer, the defect of color bleed will become much more apparent.